Electric circuit interrupting device



Jan. 26, 1943i E. A; WILLIAMS, JR., :TAL 2,309,489

ELECTRIC CIRCUIT INTERRUPTING DEVICE Filed Aug. 2, 1940 2 Sheets-Sheet l Jan. 26, 1943.

E. A. WILLIAMS, JR., ET AL ELECTRIC CIRCUIT INTERRUPTING DEVICE Filed Aug. 2, 1940 2 Sheets-Sheet 2 Fig. 9. F7@ IO. PROS/DEG 7'/ VE PROS/05677 VE CURRNT PfAK CURRENT PEAK l/I l /l l l Inventors: Edwin A.\/\/illiams Jr?,

Caid? L. Schuck, by Wav/W7 5.

Thai r Attorwey TENT OFFICE UNITED sTATEs Zdiyiii ELECTRIC CIEC'LUT NTYCRRUPTING DEVICE Edwin A.. Williams, Jr., Lansdowne, and Carl L.

Schuck, Philadelphia, Pa., assignors to General Electric Company, a corporation of New York Application August 2, 1940, Serial No. 349,847

10 Claims.

The present'invention relates to high voltage .f circuit interrupters of the kind referred to as" current limiting devices because of their ability under severe short-circuit conditions, to limit the short-circuit current to a peak magnitude considerably below that of the prospective or available current which otherwise would be produced.

There have been developed devices of this character wherein the current interrupting link or agency includes a long fusible conductive section surrounded by an inert refractory material of high dielectric strength, the form, composition and arrangement of which material and conductive section are such that, upon dissipation of the conductive section by severe shortcircuit current, there is inserted into the current path a high resistance which sharply limits the current rise and produces a rapid current decay, resulting in final circuit interruption in an extremely short period of time, not exceeding a half cycle in the case of alternating current. The sudden increase in resistance of the arc path gives rise to transient voltage surges,

the magnitude of which depends upon the current at the instant that the resistance is inserted,

as Well as upon the magnitude and rate of insertion of the resistance. It is not possible a1- together to eliminate such voltage surges, particularly with current limiting devices of high current interrupting capacity, because of the necessityfor rapid insertion of high resistance in order to produce the desired current limiting action and rapid circuit interruption. With some previous designs of current limiting devices and under some conditions of circuit interruption, however, it has been found that the surge voltages may reach undesirably high peak magnitudes which might endanger the insulation of connected apparatus.

It is an object of the present invention to provide a current limiting circuit interrupting device having an interrupting link or agency of improved construction which makes it possible to secure the desired sharp current limitation and rapid interruption of a severe over-current or short-circuit condition, and which at the same time is capable of controlling the insertion of the current limiting and interrupting resistance so as to insure against the dangerv ci occurrence of surge voltage peaks in excess of predetermined limits.

In general, an interrupting link or agency constructed in accordance with the present inverntion, embodies a plurality of elements in parallel (ICI. 20D- 120) relationship and so arranged that they function successively in a manner to cause insertion of resistance in such amounts and at such rates that there is not produced at any time during the interrupting process, a rate of current decay which will cause a surge voltage peak in excess of a predetermined magnitude.

A more complete understanding of the details of that which we consider to be novel and our present invention will be secured from the following description and the appended claims, taken in conjunction with the accompanying drawings wherein Fig. 1 shows, in longitudinal cross-section, an exemplary form of current limiting circuit interrupting device to which the present invention is applicable; i

Figs. 2, 3, 4, and 5 show details of construction oi' (iiiferent forms of current interrupting links which embody the present invention and which are suitable for employment in a device such as that shown in Fig. l;

Fig. 6 is an enlarged perspective view of a part employed in the constructions of Figs. l, 2, 3, and 4;

Fig. 'T is a longitudinal section of one end of a device such as shown in Fig. 1, illustrating the manner of incorporating therein an interrupting liniev having elements constructed as shown in Figs. 4 and 5;

Fig. 8 is an end View of the core employed in the structures shown in Figs. 1 and 7;

Fig. 9, for purposes of later description and comparison, contains diagrammatic representations oi oscillographic curves of current and voltage, respectively, illustrating conditions of circuit interruption as secured with a current limiting device wherein the current interrupting link is of a form old in the art; and

nal ferrules 2|.

Fig. 10 contains diagrammatic representations of oscillographic curves of current and voltage,

respectively, illustrating conditions of circuit interruption as secured with an interrupting link constructed in accordance with the present in' vention.

By reference to the drawings, it will be seen that the illustrated circuit interrupting device has a tubular enclosing casing 20 upon the 'opposite ends of which are disposed metallic termi- While the casing is'indicated as being constructed of glass, other suitable materials of high dielectric strength, such as porcelain, may be employedif desired. The terminal ferrules are secured on the casing by any suitable form of metaltog1ass seal, indicated at 22, and the outer ends of the ferrules are closed by sealed-n caps 23. As is common practice in devices of the type illustra-ted, an insulating and refractory core 24, extending longitudinally in casing 2G, is provided for the purpose of supporting the long current interrupting link so that the latter may be accommodated in a casing of practical dimensions. The particular interurn spirally wound on the core the form shown in Fig. 2 and includes two elements 25 and 26 in parallel relationship, vhich are described in detail hereinafter. any of the other forms of interrupting links illustrated in the drawings, may be supported the core in a similar manner. The inter of the casing 20 around the core is filled with a granular refractoryT arc-quenching matelial such es sand or pulverized quartz, which high dielectric strength and is substantially inert or nou-gas-producing, at the i, :ely to be encountered during cir- 1 `hiterruption. As will be evident from Figs. f core 2i has o. plurality of longitudinally .ung fins or ribs which engage the conrs cf the interrupting link only at spaced and serve to maintain the conductors f utwardly from the central body portion o e core. Thus, the maximum length of the conductors is embedded in and directly exposed to the arc-quenching material.

' opposite ends or" elements 25 and 26 are secured to iinals 3U which are mounted respectively on om. 'te ends of core 2l. suitable u may be employed for purrticular terminals employed in constr 'ion are oi' the improved i supplication Serial No. 347,437, no stent 2,294,132, oi C. L.

unal precaution a l 3l "ie ends of the core in a manner r the terminals in place. Tabs i rminals and are joined to terminal ierrules 2l so as to le between the terrra pe securely ."terrupting s conthe preihren elements in paraliel no and designe.. to iiuictlon successiveN eying a complete underunentals lotion will be treated as in- :.l elements. @ne of these rovide 'the main current ormai circuit conditions errupti fr process under element will i or curwill be voltage interruptement` and istance so as to p .lfzs ei;-

n the embodiment cL Fi current element, and is coi4 ble conductor of thin cross-section, which may be of any suitable form and material in accordance with known principles for the construction of lnterrupting elements for current limiting circuit interrupting devices. The conductor 33 of the illustrated construction comprises a. fine silver wire, which is particularly satisfactory for current conduction under normal circuit conditions. Smallbeads or thermal reservoirs 3l, of refractory insulating material, surround short sections of wire 33, preferably adjacent the middle thereof, for the purpose of determining the minimum current at which interruption takes place. These thermal reservoirs prevent contact of the enclosed wire sections with the granular filler material and hold in the heat. Thus it is insured that the wire will melt on an overload involving a lower magnitude of current than would be possible with the same conductor without the thermal reservoirs.

The voltage element, designated 2i in the construction of Fig. 2, comprises two conductors 33 connected at their outer ends to terminals 3|! and joined at their inner ends by an insulating refractory member 36. As will be seen best from Fig. 6, each of the conductors 35 is looped through a respective one of the openings 31 in member 33, and the conductors are secured by twisting; as indicated at 38. Member 36 maintains the ends of conductors 35 in spaced relationship so as to provide a gap which is calibrated to lash over at a predetermined voltage. The calibration of the gap is determined in a manner hereinafter pointed out and may vary for interrupting devices of different voltage and current ratings. The gap may be located at the end of a voltage element having one long conductor instead of two serially related shorter conductors. How "er, it has been found that bestresults are ol'itoir. i with the gap located adjacent the middle oi 5 f voltage element illustrated. The form and dimensions ci conductors 35 are determined in accordance with known principles for the construction of fusible ments for current limiting circuit interrupting devices, with certain variations which will be poin j out. In accordance with the present invention, conductors 35 are comprised of a material having high electric resistivity and a high temperature coeicient of resist-ance. We have found that a good material for the purpose ls a nickel-chromium-iron alloy known by the trade name Hytemcof although other suitable materials haring the above mentioned characteristics may be employed if desired.

The current element embodied in interrupting links constructed in accordance with the present invention, may comprise etiher a single unit, such as that designated 25 in Fig. 2, or a plurality of similar units connected so as to provide parallel as shown in Fig. 5. Although the inual units are designated by the numeral l0 5, they fundamentally the same in unit designated 25 in Fig. 2. ge element may comprise single unit, such as that designated 26 either a in Fig. 2, or may comprise a plurality of similar units connected in parallel branches as shown in lli-isf 4, the individual units 4I of Fig. 4 being of terrupting action is produced with fusible conductors of thin cross-section. Hence, when the current rating demands a relatively large total cross-section, it is preferable to divide this total cross-section into a number of parallel units in which the individual conductors are of such cross-section as to provide the most effective current limiting and circuit interrupting action. Il', in the practice of the present invention, it is found that the total number of units in the current and voltage elements is greater than can be conveniently connected between a single set f terminals, such as those previously designated 30, then the units of the current element may be connected between one set of terminals and the units of the voltage element connected between a separate set of terminals. For clarity, the terminals in the construction of Fig. are designated 42, while the terminals in the construction of Fig. 4 are designated 43, although both sets of terminals may be of the same form as those designated 30.

those shown in Figs, 4 and 5 are The current elements of Figs. 2 or 5, if ernn ploycrl without 'the voltage elements, respond fundamentally. to the interrupting ele ments previcaislyknown forms of current lim iting circuit interrupting devices. In order more clearly to show the advantages of employing our auxiliary or voltage element, the operation under severe short-circuit conditions of an interrupting device employing an interrupting element corresponding to the current element alone. now will be described. This operation is best explained by reference to Fig. 9, wherein are represented oscillograph curves of short-circuit interruption by means of a current limiting cire cuit interrupting device employing an interruptm ing element similar to the current element of Fig. 2, it being pointed out, however, that substantially the same character of interruption would be secured if a pluralmnit element, corresponding to the current element of Fig. 5,. were employed. The short-circuit, and overheating of the conductor, such as 33, begin at a. Substantially simultaneous dissipation of all portions of the conductor, and initiation of arcing, take place at b. When the conductor volatilizes at o the metallic vapors are dispersed extremely rapidly through the spaces between the relativelyu cool granules of the surrounding quenching material and condense on the surfaces of the granules, so that the vapors no longer are available for current conduction. Also. the intimate physical contact between the are and the cool granules causes an exceer rapid transfer ci heat from the are to the d ring material. Most of the arc energy is d in this heat trarrei ing device is practically negligible. The sub stantially simultaneous volatilization of all portions of the length of the fusible conductor, the sudden removal of the metallic vapors from the arc path, and the extremely rapid cooling of the arc, have the effect of suddenly inserting' an extremely high resistance into the current path, which sharply limits the current substanM tially to the value required to melt and volatilize the conductor. The broken lines indicate the manner in which the prospective or avait able current would develop under severe shortm effective fusible length of 'the f In case constructions such as sity the element of time is circuit conditions if the interrupting device were not connected in the circuit. It has been found that by means of the action described, a prospective short-circuit current of say 60,000 rms. amperes may be limited to a maximum peak value of between 200 and 20,000 amperes, depending upon the particular design of the interrupting device and the particular circuit conditions.

The rapid insertion of high resistance also causes a very rapid current decay from b to some point generally designated cv in Fig. 9, after which the current decays somewhat less precipitously and is nally interrupted at d, which corresponds to the zero point of the nor mal circuit voltage. The final interruption of the current is accomplished with minimum disturbance, and takes place within an extremely short period of time which does not exceed a half cycle in the case terruption, as illustrated in the drawings. The described current limiting action is produced under all conditions within the rating of the interrupting device, provided the magnitude of current required to melt and volatilize the fusible conductor is reached before attainment of the peak magnitude of prospective current. Obviously, current limiting action cannot be produced if the peak magnitude of prospective current ls reached before attainment of the mag nitude of current required to melt and volatilize the conductor.v

The sharp limitation of the current rise and the subsequent precipitous current decay produced by the sudden insertion of high resistance as previously described, gives rise to a voltage surge which is initiated approximately at the instant of vclatilization of the fusible conductor. lire peak magnitude of the surge voltage, in-

d at e, depends upon the ohmic value the resistance inserted, the value of the current at the instant of melting, and the rate of current decay, as well as -upon the constants of the circuit. With certain forms of current lim iting interrupting devices and under some conditions, the surge voltage may rise to a crest magnitude which is undesirably high and which may be above the established impulse insulation levels of connected apparatus.

ihe employment of an auxiliary voltage elein accordance with the present invention, ely prevents such'undesirably high peak des of surge voltage. The operation of uut-ing device employing our improved ci' interrupting link, is best described by rence to Fig. i0. This operation is substan- .elly the same with either a single-unit form ci" voltage element, such as that designated 26 in i'ig. or a voltage element employing a plurality of parallel connected units or branches as those designated 4| in Fig. 4. Hence the operation will be described as if the singleunit construction o1' Fig. 2 were employed. It is believed well to point out here that of necesexaggerated in the subsequent description of operation and in the curves shown on the drawings. In reality, the described stages of the interrupting process take place in periods of a few microseconds.

By reference to Fig. l0, it will be seen that the initial stage of the interrupting process ls substantially the same as that previously described in connection with Fig. 9, since during this initialstage only the current element functions, the voltage element being prevented from carrying of alternating current irlcurrent by reason ci the gap provided by member 3E. c =hort-circuit and abnormal heating of the current element commence at a. The high ddenly inserted into the current path tion of current element, sharply at and causes a rapid ousl; indicatedy the peak current value at b, corresponds substantially to the cu eduired to melt the current element and is considerably below the prospective or available 'rirrent peak. The rapid current decay ii'nn teiy rollowing dissipation ci the currar.' n ives rise to a voltage surge as intimated at f in Fig. 10. The voltage element takes no part in this initial stage of the interrupting process but is conserved, by the gap between the ends ci conductors J5, for most effective operation. during later stages in the interrupting p i. The calibration of the gap in the volti t determines the magnitude to r t "r across the termi- ;ig no Thus, by proper the voltage surge f may magnitude considerably beated e.; c in Fig. 9, which would be luit element alone were emplayed. En prac ior any particular interrupting device an is calibrated so as to pre vent the rst voltage surge f from attaining a peak magnitude in excess of the limit, or impulse insulation level, which is predetermined in accordance with the application for which the interrupting device is designed.

When the predetermined magnitude of voltage is reached and the gap breaks down, conductors 35 of the voltage element are brought into the circuit in shunt relationship to the arc lpath formerly occupied by the current element. This occurs at a point such as that indicated at g on the current curve in the upper part oi Fig. 10. The current decag7 immediately subsequent to connection oi' conductors 35 in circuit is less rapid than that represented by the portion b-g on 'the current curve of Fig. offered by conductcrs 35, though high, is less than that of the arc path formerly occupied by the current element. .as conductors 35 heat up, their resistance rapidly increases so that a further cur- V13nt reduction is produced as indicated by the tion of the current curve g-h. Conductors rally attain their melting temperature and volet-inse at h, whereupon the resistance rises at a rate and Aproduces a rapid current decay, such as indicated from h to i, and final interruption current at d. The rapid decay of current subsequent to volatilization of the voltage element produces a voltage surge as indicated at c on the voltage curve in the lower part of Fig. i0. However, during the interval between voiat ization of the current element and Volai or 'the voltage element, a considerable i ci the system energy has been dissipated, errent has reached a reduced rate of reduced value, indicated at h, to duced upon volatilization of the voltage ont is roughly proportional. Tous, the magnitude of the second voltage surge ic is not as great as that, indicated at e in Fig. 9, which would be produced ii the current element alone were employed.

We have iound that the interval between melting oi the current element and melting of the voltage element is a very important factor in de termining the me nitude of the voltage surge crest. 'if int 10, since the resistance :val is too short, the second asoaiea voltage surge will be superimposed upon the first voltage surge peak, resulting in a high surge crest. The time spacing between melting of the current and voltage elements therefore should be such that the second voltage surge is not initiated at or near the peak of the iirst voltage surge, but rather is initiated after the rst voltage surge has fallen to a value approaching the normal circuit voltage. The interval between melting oi the current element and melting of the voltage element usually should be at leastlOO to microseconds. The magnitude of the second voltage surge is determined to a large extent by the delay afforded before melting of the voltage element. Within practical limits, the greater the interval between melting of the current and voltage elements, the smaller will be the peak magnitude oi the second voltage surge. This interval can be controlled by proper calibration of the gap of the voltage element and the melting characteristics of the voltage element. Increasing the length of the gap will delay subjection of the voltage element to the short-circuit current, and thus will delay initiation of heating of the conductive portions of the voltage elements. However, as previously indicated, the calibration of the gap determines the peak of the first voltage surge, so that a limit is reached beyond which it is not desirable to increase the gap spacing. With the gap spacing determined, and with all other conditions remaining the same, variation in the magnitude of the second voltage peak may be secured by variation in the melting characteris- Itics of the conductive portions of the voltage element. This can be accomplished by employing diiierent resistance materials. Thus, if a voltage element comprised of a given resistance material does not afford a delay sufiicient to prevent rise of the second voltage surge beyond a predetermined desired limit, there may be employed a voltage element comprised of a different resistance material having such melting characteristics as to provide the required delay. However, the melting characteristics o! materials having suitable resistance characteristics, comparable to those o! the nickel-chromium-iron alloy Hytemco previously mentionedy may not aord suiilcient latitude under some conditions. Also, from a practical standpoint it is desirable to be able to employ a single material, having the proper resistance characteristics, for the volt.

age elements of a line of interrupting devices o! different current and voltage ratings. Therefore, we prefer to employ a single resistance material of the proper characteristics, and to secure the desired delay in melting of the voltage elements by proper determination of their crosssection. Cross-section is used in the sense of total cross-section, of either the single-unit voltage element construction of Fig. 2 or a. pluralunit voltage element construction such as shown in Fig. 4. If, for a given interrupting device, a voltage element of a given cross-section does not produce reduction in the surge voltage crest below the limit predetermined for the application for which the interrupting device is designed, then a voltage element of suiiiciently larger crosssection may be employed to give the desired reduction.

The required ratio of cross-section of voltage element to cross-section of current element varies for different current and voltage ratings, the higher ratios being employed for the higher current ratings. We have found that ratios of crosssection o! voltage element to cross-section of osseuse ouvrent element falling sommeliers in the range of about 1 to 4, depending upon the rating of tlie particular interrupting device, usually are sufficient to prevent surge voltages in excess of prevailing impulse insulation levels determined for the classes of apparatus with .whioh these current limiting' eiiouit interrupting devices :sie likely to be employed. It is not feasible to set forth herein speciiio exemplaiy dimensions and cross-section ratios .for the current and voltage elements, since these vary considerably with dif ferent ratings and also depend upon particular impulse insulation levels. However, with an undersiending of the fundamental teachings set forth herein and of known principles for the de sign of interrupting devices, such es described herein' one skilled in the srt may readily determine by ssieulutioits or tests the dimensions and other details of des required for propel.a coustruotion oi en intel "opting device in accordance wit-ll thei esent u fention.

oieuted, the iuneiemental con rmt-lime l ,gore desoiilled be modi" e 'various operational charac iliciitions. io'iexemple,

= oi voltage ele- .iiielent materials, et ii ele forth in the deli: application. This "using the voltage ele' so to produce un vise tney .o s of production peuk magnitude. Wf 'limited by the details ol, in the drawings, and pendeffl olaims suoli moi bodimeuts es employ tile fu; invention lleieinbofore fully Wiiut we claim es new .1, fl Letters Potent of the il; lf

l. liu im electric circuit intermoter oi "foe out rent limiting type., e i2 i ino-ls, means providing; a. circuit of goed conduetvity between said terminals und including' e fusible seotoii9 means providing a second circuit between seid terminals in parallel with, seid, fii'st circuit including," e. high i'esisteoee @motive f having e fusible section, go #eil s *o said conductive poitioii of it to prevent the latter from carrying Lumi-ent 'until the 'voltage across the geo means flies L Y determined magnitude followinumpture oi fusible section of said first oire'uit, at quenching means associated with said fusisle sese 2@ resistance leigh 5 were employed without seid second element c 5i section said first circuit eis@ controlling tiiei tesi tenue into the c resent path upon .d s o. oe fusible section of sold first ein y so to limit the surge voltage to a orest miti substantially lower than that which would 53:. :oroilimed under m the same conditions if saisi irs oiouit were emu ployed without said gun and sommi faire cuit.

2i, lin an electric ciieuit interruptor of the current limiting type, a, pair of terminals, an ele- 5 ment of good electric conductivity eonusctsd `isetween seid teminals and having a fusible srywonn a second element connected between said i; nels in parallel with seid first element :and im smiling it illisible eoncluctlve section. having high positive temperature ce its. gap means in series with fusible set ion of said second element to prevent current eonduetion by the letter until the voltsle reaches a predemwuf e iollowing rupture of the iusible section rst elementy und are* uueuohing o@ ble of producing e current limiti-ig eetion ien dissipation o eash oi sold fusible Q'eotions u deim severe -overourrent eondi- C, tions., d usp means being arranged to break domi ef voltage substantially lower than the peak magnitud oi Volto-ge surge .likelyw to ne produced unen sei/ete overcuelent dissipation oi the fusible section @aid :tiret element if the letter d means, tile isf-sible section of seid secon. ele-d ynent 'seeing construeted so es to sieste preoiitiile time intevoi leiter down of seid @up means.

eoient of im Si. In en eleotrle circuit intervenuti s devies of ll. stimulus" illeli', i, .p means a, solitude i owiaig of ol said eii'oult, seid. :uci said eon'rluetiife portion of seid sesotld t controlling the insertion oi' resistance into the current oath upon dissipation of iusitile so es to tile voltage surges produced uson .rupture sold fusible sections under severe overouilsnt sentiitions to a crest magnitude substantially lower toon the crest magnitude of voltage surge Winoo would be produced under the upon dissipation oi the fusible section of solo first olreuit if the latter were employed without i? solo means and solo second eircuit e. In eioetrio oireuit ixiteiuuptei of current limiting type, pair of terminals, an

Aelement of goed eleetrlo ool'iduetivt conneeted between said terminals out having e long' fusible section, a second element ilmeteo between seid :terminals in parallel with seid mst element end e lnoluding a lone fusible conductive settles ilovel same conditions sistivity', gap means in series tion of sait second element he letter from. carrying current tile voltage across the gap mee-ns reaches a d magnitude following rupture of erneut, and tractors' arcquenching material fusible sections of both o1' scid ap means being arranged to break s subjected to voltage a magis substentially lower than the Y of voltage surge likely to be proissipation of the fusible section of ent under severe overcurrent coneif?. first element were employed withmeans said second element, said aid second element being desection: e voltage Y e portions hevtween for preventing sai` conductive portions is subjected to voltage or" predeter- "ude following rupture of the fusible 'st mentioned element, said conhaving high resiste-.nce and c oereture ccecient of resistance 'anu refractory are-quenching ine.- embedding said conductive portions of the e c el element.

electric circuit interrupter of the cur- 'riiting t'ype, including spaced terminals, a y carrying' element connected between said nels and having a long fusible section, and c, Meguiar inert refractory arc-quenching materlel embedding seid fusible section; a voltage contr-oi element connected between said termiourallel with first mentioned element the crest magnitude of voltage surge dosed upon interruption of severe over-curl. conditions. said control element including two seriallyrelated conductive portions, at least one ol' `which conductive portions being designed to dissipate prior to dissipation of the other of the conductive portions upon subjection of 'said control element to excess current of large insigni tuile, at least seid one conductive portion having high resistance, seid control element also includen ens providing a gap in series 'with said `onductive portions for preventing current condu ti hy the control element until the gap is subjected to voltage of predetermil Ad magnitude follo'sdug rupture of the fusible section of said C t element7 and granular inert refractory urcring material embedding said fusible conductive portions of said control element.

mousse 7. In an electric circuit lnterrupter of the currentdimiting type, a pair of terminals, means normally providing a first current path of good etivity between said terminals and inclu;- rusllole section which is capable of reducing the current of the system associated with said circuit inter ollowing severe short circult so rapidly as to tend to produce a harmful surge voltage on. cid system, und means providn ing in parallel with said first current path' a sec ond current perth which carries substantially no current under normal conditions of said circuit inierrupter, said last-mentioned means being so designed as to begin to carry the system current therethrough prior to the attainment of said l harmful surge voltage on said system including resistance means varying in ohmic value in dependence upon the current passing therethrough and fusible so as finally to interrupt t current in second path.

8. In an electric circuit interrupter capable of substantially li ing the magnitude of fault our rent flowing therethrough including a. fusible element for normally provldinff a current path having good conductilty through said circuit interrupter, and ineens for limiting the transient voltage surge which results due to the currentlimiting feature of said circuit iuterrupter under severe fault conditions to a predetermined safe value comprising a circuit connected in parallel With said fusible element which normally carries substantially no current but citer fusion of said fusible element und before said voltage surge can exceed said predetermined safe velue carries the fault current through' said circuit interrupter, said l: mmentioned being fusible finally to interrupt the circuit at least 100 micro-seo cfids after the fusion of said fusible element.

n an electric circuit interrupter capable of substantially limiting the magnitude of fault current flowing therethrough including a fusible element for normally providing a current path having goed conductivity through said clrcuit interrupter, and means for limiting the transient voltage surge which results due to the current-lirniting feature of said circuit interrupter under severe fault conditions to a predetermined safe value comprising a circuit connected in parallel with seid fusible element including 9, plurality of parallel gaps which normally carry substantially no current until alter fusion of said fusible element.

l0. In an electric circuit interruptor capable of substantially limiting the magnitude of fault current flowing therethrough including a fusible element for horn .liv providing a current path having conduct... t3. through said circulfJ interruptor, and means for limiting the transient voltage surge r h re due to the currentlimiting feature of said 3u-cuit interrupter under severe fault con uic-ns to u predetermined sole value comprising e. circuit connected in 'parallel with said fusible element including a plurality of parallel gaps cf dizlerent spacing which normally carry suhstent' illy no current until uiterffusion of said fusible element.

EDWIN A. lNILLIAMS, JR. CARL L. SCHUCK. 

